An image field is a rectangular area on a reticle that has the purpose of containing pattern which is exposed in one “shot,” “flash,” or “scan” in the lithographic process with actinic radiation, thereby leading to the creation of the corresponding pattern in the illuminated exposure field on the processed wafer. The most common type of productive reticles have only one image field which often contains several identical dies with pattern for a specific chip layer. Generally, reticles with one image field will be called single layer reticles in the following. Reticles with more than one image field with pattern for different layers and/or different products are commonly called multilayer or multiproduct reticles, but will be indiscriminately referred to as multilayer reticles in this disclosure for the sake of brevity. A group of reticles containing the pattern for all layers of a chip or product—or, in the case of multiproduct reticles, several products—is called a reticle set. Commonly, all reticles of a reticle set, or at least those that are used with the same or similar types of exposure tools, have the same image field layout or geometry, i.e. the number, dimensions, and positions of the image fields are equal for these reticles. (For the sake of brevity, the possibility of differing standard image field geometries for different classes of exposure tools will not be explicitly mentioned throughout this disclosure; however, when referring to a standard or original image field layout or geometry of a reticle set in the following, these terms are to be understood as being potentially modified by the words “for each class of exposure tools” or “for the class of exposure tools of the reticle”). In the following, an image field containing pattern for a chip layer will be called a functional image field. Other image fields on a multilayer reticle will be called unused image fields. Unused image fields are often present on some reticles of a multilayer reticle set for different reasons. They may be left empty or filled with dummy pattern or other non-productive structures. When referring to the wafer exposure process for a given chip layer, the image field used will be called the actual image field. An exposure of a wafer with the actual image field, with the purpose of creating functional pattern on the wafer, will be called a productive exposure. A wafer processed with the purpose of creating functional pattern on the wafer by productive exposures will be called a productive or product wafer. An exposure with a reticle which has not the purpose of creating functional pattern on a wafer, for example with portions of the reticle not containing the full actual image field, will be called a dummy exposure. A wafer exposed without the purpose of creating functional pattern on the wafer will be called a dummy wafer.
In semiconductor fabrication and lithography, reticle costs can be reduced by combining different image fields containing patterns for different layers of one product, or for the same or different layers of different products, on one reticle. However, employing these multilayer reticles or multiproduct reticles also reduces scanner throughput due to a reduction in the size of the image field as compared to single layer reticles. Such multilayer or multiproduct reticles are therefore mostly used for low-volume products as, for example, test chips.
Reticle heating, which is essentially symmetric in ordinary single layer reticles, occurs through absorption of actinic radiation in the reticle body, and especially in the absorbing, patterned reticle surface. Asymmetric reticle heating is particularly acute in multilayer and multiproduct reticles but can also occur in single layer reticles having substantial variation in average transmission (for transmissive reticles) or reflection (for reflective reticles, e.g. EUV reticles) between different regions of the image field. Reticle heating causes thermal expansion of the reticle. While symmetric reticle heating causes symmetric thermal expansion of the reticle image field, which can mostly be compensated by the optics of the lithographic exposure tool or scanner, asymmetric reticle heating leads to asymmetric expansion. Asymmetric expansion of the reticle image field causes problems or distortions that cannot be sufficiently corrected with many types of scanner optics. Trapezoidal distortion is one of the most troublesome types of distortion. As a consequence, asymmetric reticle heating and thermal expansion can cause overlay degradation that can lead to yield or performance degradation.
Known techniques for mitigating asymmetric reticle heating would substantially slow down current manufacturing processes, increase manufacturing costs, and create technical and logistical issues including those that require redesign of current systems. For example, reducing the scan speed or introducing additional lag times gives the reticle more time for heat dissipation during the exposure sequence and thereby reduces the peak temperature and the maximum distortion of the reticle. However, since the timescale of heat dissipation of the reticle lies in the 10 minute range while the exposure time for a complete wafer is usually less than 1 minute, a significant throughput reduction is needed to achieve a significant overlay improvement. In addition to a reduction in throughput, which in turn increases costs, the longer delay time between wafer preparation and exposure can cause technical problems.
Another alternative to mitigate asymmetric reticle heating is to split up wafer lots (e.g. 12 to 25 wafers) into smaller groups of wafers (e.g. 1 to 3). The smaller split lots are then exposed with sufficient time intervals between each other to allow the reticle to cool down between two split lots. Maximum temperature increase of the reticle during exposure is therefore reduced due to the shorter exposure time required for smaller lots. This option, however, also reduces throughput and speed and requires considerable logistic effort.
A need therefore exists for methodology enabling reduction of asymmetric reticle heating and asymmetric reticle thermal expansion without significantly reducing manufacturing speed and throughput and without significantly increasing costs for multilayer reticles, multiproduct reticles, and single layer reticles having high variations in transmission or reflection.